1. Technical Field
The present invention relates to a radar device which receives a signal of a reflected wave that is reflected from a target, through an antenna to detect the target.
2. Description of the Related Art
A radar device radiates a radio wave from a measuring point, which receives a signal of a reflected wave that is reflected from a target, and which measures the distance between the measuring point and the target, the direction, and the like. Recently, particularly, a radar device which can detect not only an automobile, but also a pedestrian as a target by a high-resolution measurement using a short-wavelength radio wave such as a microwave or a millimeter wave has been developed.
A radar device sometimes receives a signal in which a reflected wave from a target at a short distance and that from a target at a long distance are mixed with each other. In the case where a range side lobe appears in the autocorrelation characteristics of a signal of a reflected wave from a target at a short distance, particularly, the range side lobe is sometimes mixed with a main lobe appearing in the autocorrelation characteristics of a signal of a reflected wave from a target at a long distance. In this case, the accuracy of detection in which the radar device detects the target at a long distance may be impaired.
In the case where an automobile and a pedestrian are at the same distance from a measuring point, moreover, a radar device sometimes receives a signal in which signals of reflected wave from the automobile and pedestrian having different radar cross sections (RCS) are mixed with each other. It is usually said that the radar cross section of a pedestrian is smaller than that of an automobile. Therefore, a radar device is requested to, even in the case where an automobile and a pedestrian are at the same distance from a measuring point, properly receive not only a reflected wave from the automobile, but also that from the pedestrian.
Therefore, a radar device which must perform a high-resolution measurement on a plurality of targets such as those described above is requested to transmit a pulse wave or pulse modulated wave having characteristics in which the autocorrelation characteristics are in the low range side lobe level (hereinafter, referred to as “low range side lobe characteristics”). Moreover, such a radar device is requested to have a reception dynamic range which is so wide that, in the reception by the radar device, signals of reflected waves reflected from targets that cause various reception levels depending on the distance and kind of a target can be received.
In regard to the above-described low range side lobe characteristics, a pulse compression radar has been known which, by using a complementary code, transmits a radio frequency transmission signal as a pulse wave or pulse modulated wave having the low range side lobe characteristics. Here, the pulse compression means that a pulse signal is pulse-modulated or phase-modulated, transmission is performed by using a signal having a wide pulse width, and, in signal processing after reception, the received signal is demodulated and converted (compressed) to a signal having a narrow pulse width, and is a method of equivalently enhancing the reception power. According to the pulse compression, the target detectable distance can be increased, and the distance estimation accuracy with respect to the detectable distance can be improved.
A complementary code is configured by a plurality of, for example, two complementary code sequences (an, bn), and has characteristics that, considering results of autocorrelation calculations of one complementary code sequence an and the other complementary code sequence bn, in the case where the results of autocorrelation calculations are added together while the delay times (shift times) τ [sec.] are made consistent with each other, the range side lobe is zero. The parameter n is n=1, 2, . . . , L. The parameter L indicates the code sequence length, or simply the code length.
A method of producing a complementary code will be described with reference to FIG. 14. FIG. 14 is a view showing an example of a procedure of producing a code sequence of usual complementary codes. As shown in FIG. 14, from the descriptions of the fourth and fifth rows, a subcode sequence (c, d) consisting of an element of 1 or an element of −1 and having a code length of L=2p−1 is generated, and, from the descriptions of the sixth and seventh rows, a complementary code sequence (a, b) having a code length of L=2p is generated. Here, one complementary code sequence a is a coupling of the subcode sequence c and the subcode sequence d, and the other complementary code sequence b is a coupling of the subcode sequence c and the subcode sequence −d.
The code sequences a, b indicate complementary code sequences, respectively, and the code sequences c, d indicate subcode sequences constituting a complementary code sequence, respectively. The parameter p defines the code length L of the generated complementary code sequences (a, b).
The characteristics of such a complementary code (complementary code sequence) will be described with reference to FIG. 15. FIG. 15 shows views illustrating the characteristics of a conventional complementary code. In the figure, (a) is a view showing results of the autocorrelation calculation of the one complementary code sequence an, (b) is a view showing results of the autocorrelation calculation of the other complementary code sequence bn, and (c) is a view showing an additional value of the results of the autocorrelation calculations of the two complementary code sequences (an, bn). The code length L of the complementary codes used in FIG. 15 is 128.
The result of the autocorrelation calculation of the one complementary code sequence an of the two complementary code sequences (an, bn) is derived in accordance with Exp. (1). The result of the autocorrelation calculation of the other complementary code sequence bn is derived in accordance with Exp. (2). The parameter R represents the result of the autocorrelation calculation. In the case where n>L or n<1, the complementary code sequences an, bn are set to zero (i.e., when n>L or n<1, an=0, bn=0). The asterisk * represents a complex conjugate operator.
                    [                  Exp          .                                          ⁢          1                ]                                                                                  R            aa                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                    a              n                        ⁢                          a                              n                +                τ                            *                                                          (        1        )                                [                  Exp          .                                          ⁢          2                ]                                                                                  R            bb                    ⁡                      (            τ            )                          =                              ∑                          n              =              1                        L                    ⁢                                    b              n                        ⁢                          b                              n                +                τ                            *                                                          (        2        )            
As shown in FIG. 15(a), in the result Raa(t) of the autocorrelation calculation of the one complementary code sequence an derived in accordance with Exp. (1), a peak exists when the delay time τ is zero, and a range side lobe exists when the delay time τ is not zero. Similarly, as shown in FIG. 15(b), in the result Rbb(t) of the autocorrelation calculation of the other complementary code sequence bn derived in accordance with Exp. (2), a peak exists when the delay time τ is zero, and a range side lobe exists when the delay time τ is not zero.
As shown in FIG. 15(b), in the additional value of the results (Raa(t), Rbb(t)) of the autocorrelation calculations, a peak exists when the delay time (or the shift time) τ is zero (hereinafter, the peak when the delay time τ is zero is referred to as the main lobe), and a range side lobe does not exist and is zero when the delay time τ is not zero. This is expressed by Exp. (3). In FIGS. 15(a) to (c), the abscissa indicates the delay time τ) in the autocorrelation calculation, and the ordinate indicates the calculated result of the autocorrelation calculation.[Exp. 3Raa(τ)+Rbb(τ)≠0, when τ=0Raa(τ)+Rbb(τ)=0, when τ≠0  (3)]
Consequently, furthermore, a pulse compression radar has been known which, as shown in FIG. 16, transmits in a time divisional manner a radio frequency transmission signal that is generated based on the above-described complementary code an, and a radio frequency transmission signal that is generated based on the complementary code bn, while switching over the signals in each predetermined transmission period. FIG. 16 is a view illustrating transmission periods Tr in a conventional pulse compression radar, and the complementary codes an, b which are used in transmission in the transmission periods. When a target moves in the case of reception in such a conventional pulse compression radar, the received reception signal is affected by a phase change θ(t) shown in Exp. (4) due to a Doppler frequency fd that is generated in accordance with the movement. The parameter t represents the time.[Exp. 4]θ(t)=2π×fd×t  (4)
In a state where an influence of such a phase change θ(t) is exerted, there arises a problem in that the range side lobe level in the autocorrelation characteristics of the reception signal is not zero, and the low range side lobe characteristics in the autocorrelation characteristics of the reception signal are not realized.
This problem will be specifically described with reference to FIG. 16. In FIG. 16, the transmission interval of transmitting the radio frequency transmission signals which are generated based on the complementary codes an, bn, respectively is set as the transmission period Tr. In this case, after the transmission of the radio frequency transmission signal which is generated based on the complementary code an, the signal of the reflected wave with respect to the radio frequency transmission signal which is generated based on the complementary code bn is received during the next transmission period Tr. However, the signal of the reflected wave undergoes the phase change θ(t) shown in Exp. 4.
Depending on the magnitude of the product of the transmission period Tr and the Doppler frequency fd contained in the reflected wave signal, therefore, the above-described ideal low range side lobe characteristics are hardly obtained, and the low range side lobe characteristics are impaired. In FIG. 16, the parameter Tp indicates the transmission time per pulse corresponding to a pulse code having the code length L. The parameter Tc indicates the transmission time in a transmission zone of the radio frequency transmission signal that is generated based on the complementary code an or bn having the code length L. Exp. (5) holds among the parameter Tp, the parameter Te, and the parameter L.[Exp. 5]Tc=Tp×K  (5)
Patent Document 1 is known in connection with the above-discussed problem, i.e., the problem in that, when the radio frequency transmission signals that are generated based on the complementary codes an, bn, respectively are switchingly transmitted in a time divisional manner, the low range side lobe characteristics is impaired due to the Doppler frequency fd.
The dispersion/compression type pulse echo system transmitter/receiver shown in Patent Document 1 transmits radio frequency signals which are modulated by code sequences of different pulse compression code sequences in accordance with modes (a B mode and a Doppler mode). Specifically, in the B mode, the transmitter/receiver transmits a radio frequency signal which is modulated by a compression code sequence for a short distance range. In the Doppler mode, the transmitter/receiver transmits a radio frequency transmission signal which is modulated by a Barker code sequence, an M sequence, or the like. According to the configuration, transmission pulses are selectively used in accordance with the measurement target, and a pulse echo caused by a fast moving target which is at a short distance can be reduced.